A problem associated with the burning of biomass fuel in general is the production of air pollutants. For example, the burning of biomass fuel (and particularly the inefficient burning of biomass fuel) may produce volatile, toxic, or other undesirable gases. Furthermore, large amounts of smoke and particulate matter may also be released into the atmosphere.
In this regard open fireplaces are particularly inefficient. That is, open fireplaces usually produce larger amounts of air pollutants, as compared to enclosed fireplaces. Furthermore, an open fireplace generally only provides heat directly in front of the fireplace, with the vast majority of the heat being lost up through the chimney or out through the rear wall of the fireplace.
The inefficiency of open fire places has been addressed to a certain extent by the use of domestic furnaces. Examples can be found in U.S. Pat. No. 4,559,882 (Dobson) and U.S. Pat. No. 4,630,553 (Goetzman).
However, whilst the problems of the inefficient burning of biomass fuel for space heating can be addressed somewhat with a furnace, the extra capital cost is not always necessary, practical, or affordable. Moreover, furnaces are usually closed off from view, and do not therefore provide the psychological or aesthetic benefits that are derived from seeing (or hearing) flames.
Perhaps as a result, domestic wood burners have become increasingly more popular over the years, and are now in widespread use. “Wood Stove” is another common term for such appliances, particularly in North America.
Wood burners generally comprise a metal firebox, into which biomass fuel may be placed and burnt, an adjustable air control or damper, and an exhaust flue. Many, if not most, wood burners also have a glass door through which the fire and/or flames may be viewed.
The combustion of biomass fuel is a complex process, and includes a range of chemical reactions.
The main stages associated with combustion are drying, pyrolysis, combustion and reduction, which if done correctly produce the combustible gases carbon monoxide and hydrogen (along with small amounts of other gases such as methane). The carbon monoxide and hydrogen can then be combusted separately during what is known as secondary combustion to yield water and carbon dioxide (and heat).
However, most wood burners lack the ability to adequately combust or convert the combustible gases (during secondary combustion) due to the wood burner not being able to produce and/or maintain high enough temperatures to do so.
That is, the temperature required to combust or convert most of the various combustible gases (and/or particulate matter) is between 500-800° C.
In general terms, to ensure complete combustion of the biomass fuel, and any resulting combustible gases and/or particulate matter, the fire needs to reach and consistently maintain a temperature of 750° C. or greater (for the entire flame path). And whilst many prior art wood burners can attain such a temperature at times, they lack the ability to consistently maintain the fire above this temperature—resulting in periods of incomplete combustion.
U.S. Pat. No. 4,672,946 (Craver) describes a wood burner which has a secondary combustion means for burning the particulate matter in the flue gases. However, the temperature reached within the firebox of the Craver device is stated as being only around 540° C. (1000° F.) and the secondary combustion region only reaches up to around 760° C. (1400° F.). Hence, a disadvantage associated with Craver is that the wood burner is not able to maintain temperatures high enough to consistently combust or convert the combustible gases and/or particulate matter.
It may be of advantage therefore if there was a combustion system which had the ability to reach and maintain temperatures high enough to consistently combust or convert the combustible gases and/or particulate matter.
In recent times, many countries or local bodies have introduced regulations to restrict the sale of inefficient and/or polluting wood burners.
For example, in New Zealand the generally allowable standard for wood burners is a maximum of 1.5 grams of particulate matter released per kilogram of wood burned, accompanied by a minimum efficiency of 65%. However, some regions have gone further than this. For example, the Canterbury Regional Council in New Zealand (which is in the region of a weather-inversion layer) has lowered these levels to 1.0 grams of particulate matter per kilogram of wood burned. The Regulations further restrict the use of wood with a moisture content higher than 25%.
However, these Regulations are not retrospective, and hence they only have effect in relation to wood burners manufactured and sold after the Regulations came into force. Moreover, to date there have been no innovations which have enabled people to bring their older wood burners up to modern compliance levels (voluntarily or otherwise).
It may be of advantage therefore if there was available a combustion system which was able to be retro-fitted to an existing wood burner, for example to increase its efficiency and/or to bring it up to modern compliance standards.
Two factors which usually have the most detrimental effect regarding the efficiency of, and/or the release of air pollutants from, a wood burner are to do with refueling the wood burner and when shutting down or reducing the air supply to the wood burner.
Refueling causes quenching, a situation where the introduction of fresh fuel to the fire is not supported by the heat contained within the existing fire to adequately pyrolyse the biomass. As a result, visible smoke and particulate matter are often seen exiting the top of the flue or chimney at this time. This can take a while to subside as enough heat builds up in the fire to commence the correct chemical processes required to efficiently and/or completely combust the fresh fuel.
Hence, a common problem associated with existing wood burners is that during times of refueling, and the resultant quenching, the temperature within existing wood burners decreases significantly, and to temperatures below that at which the wood burner is able to efficiently or fully combust or convert the combustible gases and/or particulate matter—as described previously.
A wood burner user may wish to reduce the air supply to keep the fire burning longer and/or while they are asleep. This is known as “banking”. In doing so, they generally place a full load of biomass fuel in the wood burner and shut down (or minimise) the air supply to prolong the burn time. However, the reduction in available oxygen and the corresponding detrimental effect on combustion results in more air pollutants being produced and released. Because this often results in the amount of air pollutants exceeding the minimum regulated amounts, many modern wood burner designs have denied the user the ability to shut down the air supply.
The air supply also affects the dynamics of wood burners because a greater draught causes more heat to be generated, but a greater portion of heat is lost up the flue. The higher velocity of gases also results in more particulate matter being exhausted to the atmosphere.
Conversely, a lesser draught reduces the amount of particulate matter being exhausted from the combustion chamber but also reduces the heat output. However, although less heat may be generated, less heat is also lost to the atmosphere as the heat has more time to radiate off before being exhausted.
Or to put it another way, increased air supply means greater heat, but lower efficiency, however the greater heat actually results in a cleaner burn which lowers the emissions. With a lesser air supply, the fires get greater efficiency but the lower heat increases emissions.
It may be of advantage therefore if there was available a combustion system which was both hot and efficient, resulting in both greater heat and fewer emissions.
Many existing wood burners have a primary combustion zone for pyrolysising and/or combusting the biomass fuel, and a secondary combustion zone for subsequently combusting the combustible gases and/or particulate matter produced from the pyrolysis and/or combustion of the biomass fuel.
Such wood burners often introduce a secondary air supply into the wood burner, in the region of the secondary combustion zone, designed to add extra oxygen and/or create turbulence, in an attempt to aid combustion. Examples of such wood burners include U.S. Pat. No. 4,856,491 Ferguson et al, U.S. Pat. No. 4,832,000 Lampa, U.S. Pat. No. 4,854,298 Craver and US 2011/0005509 Marple.
However, a disadvantage associated with such wood burners is that the introduction of the turbulent secondary air serves to increase the air supply (and oxygen content), resulting in greater heat but lower efficiency, as described above. Furthermore, the secondary air supply is not heated, and it therefore has the effect of quenching the fire when it is first introduced.
WO 2012/150868 Stewart describes a combustion system which includes a secondary air supply means which introduces heated secondary air into the region of the secondary combustion zone, with the secondary air supply being sourced from within the firebox. An advantage of such a system is that the secondary air supply means takes already-hot air from within the firebox, and super heats it, before introducing it into the secondary combustion zone (where the very high temperature of the air—not the oxygen content or turbulence—greatly assists combustion in the secondary combustion zone).
Whilst the combustion system described in Stewart is very effective, the oxygen content of the secondary air is minimal given that the secondary air is sourced from within the wood burner. Furthermore, the secondary air is heated by exhausting flue gases and the primary combustion zone, whereas the hottest region of the fire, and therefore perhaps the best region to heat the incoming secondary air, is the charcoal/reduction layer formed below the primary combustion zone and/or the region of the secondary combustion zone.
Having regard to the foregoing, it may be advantageous if there was available a relatively simple and/or improved combustion system, which included primary and secondary combustion zones which were able to result in the more efficient combustion of biomass fuels and/or result in a lesser amount of air pollutants being released, as compared to presently available or prior art combustion systems or wood burners.